Criminal courts throughout England and Wales face widespread disruption this week as legal aid defence solicitors stage a new wave of protests against proposed changes to the way their services are paid for. More than 1,000 are expected to take part in a mass lobby of parliament this afternoon as a prelude to three days of working to rule in magistrates courts around the country.

The protest, backed by pressure groups including the Howard League for Penal Reform, comes as the Law Society is poised to announce whether it will challenge the new legal aid contract in court.

The solicitors' professional body is expected to go ahead with the challenge on the basis that the contract is unfair and unenforceable, as the Legal Services Commission (LSC), which administers legal aid, has the power to change it unilaterally.

The work to rule will take a variety of forms but a typical tactic is for just one defence solicitor to turn up to cover all the law firm's caseload for a given day. The move is expected to cause huge disruption to the work of the courts. It comes at a time when courts are already hit by a work to rule by administrative staff, members of the Public and Commercial Services Union, who plan to step up their action in a protest over low pay and job cuts.

The solicitors are fighting government proposals for a major overhaul of the legal aid scheme based on proposals from a government-commissioned review by Lord Carter of Coles. Solicitors will be paid fixed fees instead of hourly rates for handling a case - which they say will mean at least a 10% reduction on already low pay rates - before moving to a system of price-competitive tendering.

The lawyers say the reforms would close up to 1,700 law firms and threaten the collapse of the criminal defence system. Roger Peach, chairman of the Criminal Defence Solicitors Union, said: "The government intends to drive hundreds of legal aid firms out of business. This monopoly purchaser is performing an experiment without precedent, which will leave tens of thousands of people unable to access justice."

Greg Powell, president of the London Criminal Courts Solicitors Association, said: "We are demonstrating because we are at a loss to know how to make the Government listen to why their reforms will inflict irreparable damage to our justice system. There is an unfolding and unprecedented crisis in legal aid supply."

The solicitors accuse the government of ignoring a report by Otterburn Legal Consulting, commissioned by the LSC, which warned of the "highly fragile" financial position of many legal aid defence firms and urged the commission to delay the fee cuts. The report was received by the LSC last November but not handed over to the Commons constitutional affairs committee, which is inquiring into legal aid, before it took evidence, and was published only this month.

A spokesman for the Department for Constitutional Affairs said: "It is unprofessional and irresponsible for solicitors to abandon their court responsibilities to go on a rally. Any disruption to the courts is against the public interest."